Alterations of pulmonary arterioles have been thought to cause right ventricular hypertrophy in infants with coarctation of the aorta. To investigate the pulmonary vasculature in coarctation of the aorta, we created postductal coarctation of the aorta in utero in 23 fetal lambs at 10 to 11 weeks of gestation. Comparing the experimental lambs at term (21 weeks) to 13 months postnatally with 36 age matched normal lambs, we found that pulmonary artery pressures and right ventricular weights were evaluated in those lambs with coarctation. Similarly, pulmonary arteriolar medial thicknesses were statistically significantly increased in lambs with coarctation as compared to normal lambs.